Let an edge pulse end where it started, and draw what it carried

The pulse named its own end colour, so a selected edge decayed to the
neutral stroke and snapped back to blue — a second pulse. Dropping the
`to` keyframe lets the animation land on whatever the edge rests at.

Clicking an edge now shows the same sparkline the node panel draws for
that message, so a single value is read against how it has been moving.

frontend/src/components/Flow/flow.css

@@ -37,15 +37,16 @@
     animation: edge-pulse var(--duration-pulse) var(--ease-emphasized);
   }
 
+  /*
+   * No `to` on purpose: the implied one is the edge's own resting style, so
+   * the decay lands exactly where it started rather than on a second colour
+   * that has to snap back — and a selected edge decays to its own blue.
+   */
   @keyframes edge-pulse {
     from {
       stroke: var(--primary);
       stroke-width: 2.5;
     }
-    to {
-      stroke: var(--xy-edge-stroke);
-      stroke-width: var(--xy-edge-stroke-width);
-    }
   }
 }